1. Field of the Invention
This invention relates to a servo-control method and servo-controller of a storage device, as well as the storage device, for reading data on a track of a storage medium by way of a head, and more particularly to a servo-control method and servo-controller of a storage device and the storage device for positioning the optical beam of the head at a target position.
2. Description of the Related Art
Memory storage devices that use storage media such as optical disks or magnetic disks are widely used. For example, as optical-disk devices there are CD players, CD-R drives, or magneto-optical drives. In reproduction devices or recording/reproduction devices that use these kinds of storage media, it is necessary to position the reading position of the head at a target position.
There is a servo-control unit installed in a memory storage device that uses this kind of medium for positioning the reading position of the head at a target position. This servo-control unit detects an error signal that indicates a position deviation between the reading position of the head and the target position, and then controls the reading position of the head such that it follows the target position. An example of using this kind of servo control in a magneto-optical device, that is used as the external memory storage device of a computer, is explained.
In a magneto-optical disk device, a magneto-optical disk medium is used for storing data. This disk medium comprises a substrate, and recording layer that is formed on top of the substrate and that is made of a magnetic material. This medium is recorded information by using the change in heat and magnetic field due to light. There is a data track on this medium for recording and reproducing the data. Generally, a spiral shaped groove (tracking groove) is formed on the top of the substrate of the disk medium. The recording/reproduction track for data is formed on the land between grooves.
The optical beam from an optical head tracks this track. Information is recorded on the track during recording by using the change in heat and magnetic field that occurs due to light. When reproducing the information, magneto-optical effect is used to reproduce the information from the reflected light of the light beam. To record and reproduce information, a laser beam from a laser diode passes through the objective lens of the optical head and is focused on the surface (recording surface) of the disk medium. This focused state must be maintained, as well as the just-focused state. The servo control required to do this is called focus-servo control.
Moreover, in order to record data on or reproduce data from that data track, the laser beam in the just-focused state must follow the data track. This servo control is called track-servo control.
Furthermore, together with track-servo control, the VCM for driving the carriage is controlled by servo control in order to maintain the track actuator position in the center position of the carriage (optical head). This is called carriage-servo control.
Also, when seeking, the track actuator is controlled by servo control in order to lock the objective lens of the optical head in the center of the carriage. This is called lens-lock-servo control.
A prior servo-control system is explained using the focus-servo system. Operation is basically the same for track-servo control, carriage-servo control and lens-lock-servo control.
The optical head receives the reflected beam of the light beam that is irradiated on to the optical disk. The focus-error detection circuit generates a focus error signal (FES) that indicates the amount of shift from the focus point in the focus direction from the reflected light. The focus-servo control generates a focus-drive signal from the focus-error signal to do away with the shift of the focus position. The focus actuator is driven by this focus-drive signal and moves the objective lens of the optical head in the focus direction to make the focus position of the light beam follow the recording surface.
A PID filter is used for generating the focus-drive signal from the focus-error signal. The PID filter calculates a signal that is proportional to the error signal (P component), a signal that is integrated from the error signal (I component) and a signal that is a differential of the error signal (D component), and adds the calculated results of each of these, the P component corresponds to a pass filter, the I component corresponds to a low-pass filter (LPF) for performing low-region compensation, and the D component corresponds to a high-pass filter (HPF) for performing phase compensation.
The result of low-region compensation and phase compensation by the PID filter is output as an instruction value by the power amp to drive the focus actuator. By employing this kind of low-region compensation and phase compensation, stable feedback control is possible.
However, in the high region, the phase characteristics lag due to the power amp characteristics and coil inductance of the focus actuator. Therefore, the current resulting from the calculated result (instruction value) does not flow as is to the focus coil, and the focus coil is driven by the dull rising current. Similarly, the same occurs in the track-servo system, carrier-servo system and lens-lock-servo system.
Therefore, when high-frequency noise is generated in the focus-error signal due to medium defects or external vibration, the servo system tries to follow that noise as well, so an instruction value that causes a large current to flow is output. The actual current that flows in the focus actuator in response to this instruction value lags due to reasons described later. For example, a large current flows on one side due to the first instruction value in response to the noise. Then, since the response to instruction values after that are slow, the focus system does not stabilize leading to the off-focus state.
In order limit this kind of large current, a method of limiting the current flowing in the focus actuator only during seeking (when crossing a track) has been proposed (for example, Japanese Unexamined Published Patent S61-177644).
However, the prior art had that following problem.
The prior method, limited the entire current and by large current not flowing, it prevented the focus coil from burning. In this method, by limiting the entire current, it is possible to prevent phase lag from equivalently appearing in the instruction value. However, since the low region is also limited, there is a problem in that the ability to follow low-region position shifts greatly decreases. For example, the ability to follow surface diffraction of the optical disk in the focus direction decreases. The prior method limits the current value only during seeking (crossing tracks), so the ability to follow did not become a problem.
The objective of this invention is to provide a servo-control method and servo-controller for a storage device, as well as a storage device for equivalently restraining phase lag in the high region while maintain follow-ability in the low region.
Another objective of this invention is to provide a servo-control method and servo-controller for a storage device, as well as a storage device for preventing over responding to noise and high-frequency component.
A further objective of this invention is to provide a servo-control method and servo-controller for a storage device, as well as a storage device for keeping the margin for noise and high-frequency component while maintaining the follow-ability of surface diffraction and eccentricity of the optical storage medium.
This invention provides a servo-control method of a storage device with a head that reads information from a storage medium comprising: a step of detecting an amount of position shift between a read position of the head and a target position, a step of performing PID calculation of the detected position shift, and a step of driving an actuator for positioning the read position of the head at the target position based on the PID calculation result. The PID calculation step includes a step of limiting the calculated result of the differential component of the PID calculation to a specified value.
In this invention, takes into consideration the problem of phase lag in the high region. Therefore, in order that phase lag does not equivalently appear in the high region of the servo system, the high-region component, or in other words, the calculation results of the differential component of the PID calculation, is limited to a specified value. By doing this, it is possible to maintain the margin for noise or high-frequency component even for a system with large high-region phase lag. Moreover, since the low-region component, or in other words the calculation results of the integral component, is not limited, it is possible to prevent a decrease in follow-ability in the low region. Therefore, it is possible to prevent phase lag in the high region without a decrease in follow-ability.
In another feature of this invention, the drive step comprises a step of driving an actuator for positioning a light beam from the head at a target position. A servo system is applied for positioning the light beam at a target position on the optical storage medium, so it is possible to prevent phase lag in the high region without a decrease in follow-ability even for an optical medium with large surface diffraction or eccentricity.
In another feature of the invention, the detection step comprises a step of detecting the position shift of the light beam from the focus position on the storage medium, and the drive step comprises a step of driving a focus actuator for focusing the light beam on the recording surface of the storage medium.
This is applied to the focus-servo control of the light beam, so it is possible to prevent phase lag in the high region without decreasing follow-ability, even for an optical storage medium with large surface diffraction.
Moreover, in another feature of the invention, the detection step comprises a step of detecting position shift of the light beam from the center of the track of the optical storage medium, and the drive step comprises a step of driving the track actuator that makes the light beam follow the track of the storage medium.
This is applied to the track-servo control of the light beam, so it is possible to prevent phase lag in the high region without decreasing follow-ability, even for an optical storage medium with large eccentricity.
In yet another feature of the invention, the detection step comprises a step of detecting the position of a track actuator that makes the light beam follow the track on the storage medium, and the drive step comprises a step of driving a carriage actuator for driving a carriage having the track actuator.
This is applied to the carriage-servo control of the light beam, so it is possible to prevent phase lag in the high region without decreasing follow-ability, even for an optical storage medium with large eccentricity.
In another feature of the invention, the detection step comprises a step of detecting the position of a track actuator that makes the light beam follow the track on the storage medium, and the drive step comprises a step of driving the track actuator.
This is applied to the lens-lock-servo control of the light beam, so it is possible to prevent phase lag in the high region without decreasing follow-ability of the lens position control.
In another feature of the invention, the PID calculation step further comprises a step of limiting the calculation results of the proportional component of the PID calculation to a specified value.
When it is not possible to sufficiently control the phase lag in the high region by limiting only the differential component, the high-region phase lag is compensated by limiting the proportional component as well.
Still another feature of the invention comprises a step of executing the limit step, when the light beam is being controlled to follow the track on the storage medium. The limiting process limits the output of the instruction value and follow-ability decreases a little. Therefore, the limiting process is limited to the steady state during tracking. Also, limiting process is prohibited when seeking and good follow-ability is required, when focusing, and when the tracking servo is off. This makes it possible to maintain stability even when seeking.
In one feature of the servo-control device of this invention comprises: an actuator for positioning a reading position of the head at a target position, a detection circuit for detecting the amount of position shift between the reading position of the head and target position, a control circuit for performing PID calculation for the detected amount of position shift, and a drive circuit for driving the actuator based on the calculated results. The control circuit limits the calculation results of the differential component of the PID calculation to a specified value.
In this form of the invention, it is possible to prevent phase lag in the high region without decreasing follow-ability.